Fonseca to me doesn't look like he will be the guy, his movement and athleticism isn't good enough.
If the injury doesn't hold him back Fils looks to have the most potential as he has the all round game including movement.
Shelton will never be the guy IMO, backhand is a weak shot and his forehand is erratic also. Just don't see him ever getting to that level. His ceiling is capped in my view
I like Tien has a high IQ but he may ultimately be too small to be on that level. A promising talent though.
In order for Ben Shelton to have a chance to win slams, he has to go full Servebot
Basically a serve as good as Goran Ivanišević, Sampras, Del Poltro, etc
He can't play that long rally baseline game when you got guys like Sinner, Alcaraz, or even Zverev that can pick you apart if you attempt that. He needs to find ways to shorten points. Work on his forehand. Just play the pure power game. I don't want him to go away from his strengths to be "more well-rounded".
If Shelton can win 2 Grand Slams in his career, that would be a success IMO